Isolated DC/DC (for direct current/direct current) converters can have zero voltage switching or ZVS or zero current switching or ZCS that allow a reduction in switching losses during voltage conversion. These converters are therefore particularly advantageous in an automotive application, where the energy resource is limited. In a vehicle, a voltage converter can be used to adapt voltage levels between a plurality of electrical systems in the vehicle or to convert a voltage between an energy source and an electrical load on board the vehicle.
An isolated DC/DC converter is known from the U.S. Pat. No. 5,754,413, which is illustrated in FIG. 1. The converter comprises two switches Q1, Q2 that have their centre point connected to a branch that comprises two transformers in series. The converter is designed as a half-bridge. The switches control the transmission of energy through the transformers in order to obtain conversion of an input voltage of the converter into an output voltage. Diodes connected to the secondaries of the transformers allow the output signal to be rectified. The output voltage is obtained by controlling the duty cycle of the switches. By modifying the duty cycle in order to attain a target output voltage value, the gain of the converter is adjusted in order to attain the target output voltage value. In particular, when the input voltage of the isolated DC/DC converter varies, it is known practice to vary the duty cycle of the switches of the isolated DC/DC converter in order to regulate its output voltage, that is to say to maintain its output voltage at a desired value.
However, the voltage stress of the rectification diodes is dependent on the duty cycle of the switches of the converter. This stress can become great when the duty cycle becomes close to 0% or 100%. In order to limit the voltage stress of the rectification diodes, the two transformers have respective transformation ratios provided, which are different. This complicates the design of the converter, however, because the transformers cannot be identical and the current in the secondary exhibits discontinuities.
Moreover, when working with a variable duty cycle, current ripples, at the output can vary greatly, bringing about a variation in the output from the converter. In order to preserve operation with a good level of output, the duty cycle must vary little. Now, in a vehicle, the voltage of an energy source, such as a battery, can vary greatly according to the energy that is available. Such variation at the input of the converter involves the duty cycle being varied in corresponding fashion, which limits the use of the isolated DC/DC converter in a vehicle.